Currently, an optical fiber used in a popularized optical fiber transmission system has a configuration in which the outer circumference of a core is covered with a clad, and an optical signal is propagated in the core to cause information to be transmitted. Recently, along with the popularization of the optical fiber transmission system, the amount of information to be transmitted by the optical fiber drastically increases. In response to such an increase in amount of information to be transmitted, as many as tens to hundreds of optical fibers are used in the optical fiber transmission system to perform large-capacity long-distance optical communication.
In order to reduce the number of optical fibers in the optical fiber transmission system, it is known that a multicore fiber, in which the outer circumferences of plural cores are covered with a clad, is used to transmit plural signals by light propagated in the respective cores.
Non-Patent Document 1 shown below describes such a multicore fiber. However, according to Non-Patent Document 1, in an optical fiber in which plural cores are formed in a clad, there is a case in which the cores are optically connected to one another, which results in mutual interference among optical signals propagated in the cores. It is effective ways to restrict such interference of the optical signals are enlarging the distances among the cores to prevent optical connection among the cores, heightening the relative refractive index difference against the clad to concentrate light on the cores, and giving the adjacent cores largely different relative refractive index differences to restrict optical connection among the cores. Further, according to Non-Patent Document 1, in a case of a multicore fiber using plural cores whose relative refractive index difference against the clad is 0.35%, optical interference can be restricted to be small when the center-to-center distance of the cores is 70 μm or larger, and in a case where the adjacent cores have largely different relative refractive index differences, optical interference can be restricted to be small when the center-to-center distance of the cores is 35 μm or larger. Such reduction in center-to-center distance enables 7 cores to be arranged in a normally used 125 μm optical fiber (Non-Patent Document 1).